1. Field of the Invention
The present invention relates to a two-stroke engine.
2. Description of the Related Art
In a known two-stroke engine, the valve opening of the intake valve, which is located on the exhaust valve side, is masked by the masking wall for the entire time for which the intake valve is open, and air discharged from the mechanically driven supercharger flows into the combustion chamber via the valve opening of the intake valve, which is located on the opposite side of the exhaust valve, and thus a loop scavenging operation is carried out (see Japanese Unexamined Patent Publication No. 2-49923). In this two-stroke engine, as illustrated in FIG. 10, the exhaust valves open earlier than the intake valves, and the exhaust valves close earlier than the intake valves
Namely, in this two-stroke engine, the intake valves open after the exhaust valves open to feed air into the engine cylinder after a blow down of the burned gas, i.e. an abrupt discharge of the burned gas, occurs. Further, the intake valves are closed after the exhaust valves are closed to feed air discharged from the mechanically driven supercharger into the engine cylinder after the exhaust valves are closed. In practice, however, when the exhaust valves are closed, and accordingly, the discharge of the burned gas is stopped, the inflow of air into the engine cylinder is also stopped, and if the intake valves remain open thereafter, air or burned gas in the engine cylinder is forced into the intake ports via the intake valves, due to the upward movement of the piston.
If air flows within the combustion chamber in a loop manner, a swirl motion is created in the combustion chamber. This swirl motion is gradually strengthened during the time for which air is fed into the engine cylinder via the intake valves, and an attenuation of the swirl motion begins when the inflow of air into the engine cylinder is stopped. If the intake valves are open after the exhaust valves are closed, however, since a part of the swirling air or the swirling burned gas is forced into the intake ports as mentioned above, the swirl motion is abruptly attenuated, and as a result, a problem occurs in that it is impossible to sufficiently promote the vaporization of fuel fed into the engine cylinder and to obtain a stable combustion by increasing the burning speed due to the swirl motion.